Synthesis and crystal structure of catena-poly[cobalt(II)-di-μ-chlorido-μ-pyridazine-κ2 N 1:N 2]

In the crystal structure of the title compound, the cobalt cations are octahedrally coordinated by pairs of μ-1,1-bridging chloride anions and bridging pyridazine ligands and linked into chains propagating along the crystallographic b-axis direction.


Chemical context
Mono-periodic coordination polymers have always attracted much interest because of their versatile physical properties (Leong & Vittal, 2011;Mas-Balleste ´et al., 2010;Cerna ´k et al., 2002;Chen & Suslick, 1993;Khlobystov et al., 2001).This includes mono-periodic coordination polymers, which can show a variety of different magnetic properties including single-chain magnetic behavior (Lescoue ¨zec et al., 2005;Rams et al., 2020;Werner et al., 2015;Sun et al., 2010;Dhers et al., 2015).In this context, of interest are coordination polymers based on transition-metal halides in which the metal cations are linked by pairs of �-1,1-bridging halide anions into chains.The most prominent cations such as Mn II , Fe II , Co II or Ni II are mostly octahedrally coordinated, which means that for the synthesis of compounds with chain structures, mono-coordinating ligands must be used and several such compounds have already been reported in the literature (Foner et al., 1975(Foner et al., , 1978;;Qin et al., 2015;Zheng et al., 2010).Compounds with a chain structure may also be observed if ligands such as tetrazole or pyridazine derivatives are used, in which the nitrogen donor atoms are adjacent.In this case, the chain structure remains unchanged and the co-ligand bridges two neighboring metal cations within the chain (Ivashkevich et al., 2009;Masciocchi et al., 1994;Thomas & Ramanan, 2016).In this context, Masciocchi and coworkers have reported on compounds with the composition NiX 2 (C 4 H 4 N 2 ) where C 4 H 4 N 2 = pyridazine (1,2-diazine) with X = Cl, Br that were structurally characterized by X-ray powder diffraction (Masciocchi et al., 1994).In these structures, the Ni II cations are linked by pairs of halide anions into chains and within the chains, neighboring Ni II cations are additionally bridged by the pyridazine ligands.
We are also interested in coordination polymers in which the metal cations are linked by small-sized anionic ligands into one-or two-dimensional networks.In the beginning, we investigated compounds based on Cu I cations and halide anions with additional N-donor coligands because we have found that, upon heating, they lose their coligands in a stepwise manner and transform into new coligand-deficient compounds that show condensed copper-halide networks (Na ¨ther & Jess, 2004;Na ¨ther et al., 2001Na ¨ther et al., , 2007)).Later we found that this synthetic procedure can also be used for compounds with divalent cations such as Cd II (Na ¨ther et al., 2017).In the course of this project, we also became interested in metalhalide compounds with paramagnetic metal cations and as part of these investigations, we reacted CoCl 2 with pyridazine and obtained a compound with the composition CoCl 2 (C 4 H 4 N 2 ).No entry was found in the Cambridge Structural Database (CSD, version 5.43, last update March 2023; Groom et al., 2016) and therefore this compound was characterized by single-crystal X-ray diffraction.Later we found that this compound had already been characterized by X-ray powder diffraction and it was concluded that it is isotypic to its Ni II counterpart (Masciocchi et al., 1994), which we found is the case.

Structural commentary
The reaction of CoCl 2 •6H 2 O with pyridazine in water in a sealed vessel at 388 K leads to the formation of single crystals of the title compound CoCl 2 (C 4 H 4 N 2 ).This compound is isotypic to its Mn II , Fe II and Ni II analogs with chloride and bromide as counter-anions, already reported in the literature (Masciocchi et al., 1994).The asymmetric unit consists of one cobalt(II) cation located at (1/4, 1/4, 1/4) on the intersection point of a twofold screw axis and a mirror plane (Wyckoff site 4c, site symmetry 2/m), as well as one chloride anion at (1/2, y, z) that is situated on a mirror plane on Wyckoff site 8h.The asymmetric unit also contains half a pyridazine ligand with all atoms located at (x, 1/4, z) on Wyckoff position 8i (m site symmetry): the complete C 4 H 4 N 2 ligand is generated by a second mirror plane at x = 1/2 (Fig. 1).The Co II cations are octahedrally coordinated by four chloride anions and two pyridazine ligands and from the bond lengths and angles, it is obvious that the octahedra are slightly distorted (Table 1).The Co II cations are linked by pairs of �-1,1-bridging chloride anions into chains that propagate in the b-axis direction (Fig. 2).The pyridazine ligands also act as bridging ligands, each connecting two neighboring Co II cations.Within the chains, all of the pyridazine ligands are coplanar.The intrachain Co� � �Co distance is 3.3443 (3) A ˚.
Figure 2 Fragment of a [010] polymeric chain in the title compound.

Figure 1
Crystal structure of the title compound with labeling and displacement ellipsoids drawn at the 50% probability level.Symmetry codes for the generation of equivalent atoms:

Supramolecular features
In the crystal structure of the title compound, the chains propagate in the b-axis direction and are arranged in such a way that neighboring pyridazine ligands are perfectly stacked onto each other, forming columns along the crystallographic a axis (Fig. 3).The angle between two neighboring pyridazine ligands is 180 � and the distance between their centroids is 3.6109 (1) A ˚(slippage = 0.264 A ˚), indicating �-� stacking interactions.One very weak C-H� � �Cl hydrogen bond (Table 2) is observed.

Database survey
Many compounds of the general formula MX 2 (C 4 H 4 N 2 ) (M = transition metal and X = halide anion have already been reported in the Cambridge Structural Database but there are no hits for cobalt.The compounds with NiCl 2 (CSD refcode POPCIG) and NiBr 2 (POPCOM) were structurally characterized by Rietveld refinements using laboratory X-ray powder diffraction data (Masciocchi et al., 1994).In this contribution, the compounds with Mn, Fe, Co, Cu and Zn with chloride and bromide as anions were also synthesized and from their powder patterns, the lattice parameters were determined, which indicate that the compounds with Mn, Fe and Co are isotypic to the Ni compounds; this is not the case for the compounds with Cu and Zn (Masciocchi et al., 1994).

Physical characterization
The experimental powder pattern of the title compound agrees closely with that calculated from the single crystal data, which proves that a pure compound was obtained (Fig. S1).The thermal properties were investigated by differential thermoanalysis and thermogravimetry (DTA-TG) under an air atmosphere.Upon heating, only one mass loss is observed, which is accompanied by an endothermic event in the DTA curve (Fig. S2).The experimental mass loss of 27.4% is much lower than that calculated for the removal of one pyridazine ligand (38.2%), indicating that the pyridazine ligands are not completely removed.This is supported by the fact that the TG curve still decreases upon further heating.In the DTA curve, a successive endothermic and exothermic event is observed, which points to the decomposition of this compound (Fig. S2).Symmetry code: (iii) x À 1 2 ; y À 1 2 ; z þ 1 2 .
The title compounds were also characterized by magnetic measurements.The temperature dependence of the susceptibility was measured in the range 2-300 K under an applied magnetic field of 1000 Oe.Upon cooling, a maximum is observed at 3.0 K, indicating an antiferromagnetic transition (Fig. S3).The data were analyzed using a Curie-Weiss law, leading to a magnetic moment of 5.0 m B , which is higher than expected for a Co II cation in a high-spin 3d 7 configuration.The Weiss constant of À 8 K suggests predominant antiferromagnetic interactions, but it must be kept in mind that these values are frequently too high because of the strong spin-orbit coupling of Co II .Additional field-dependent measurements at 2 K indicate metamagnetic behavior with no saturation even at high fields, as previously observed for the linear chain compound CoCl 2 (C 4 H 4 N 2 ) 2 (Fig. S4; Foner et al., 1975).

Synthesis
CoCl 2 •6H 2 O and pyridazine were purchased from Sigma Aldrich and pyridazine from Alfa Aesar.All chemicals were used without further purification.
Pink-colored crystals were obtained by the reaction of 1 mmol of Co(NCS) 2 (237.9 mg) and 1 mmol (72 ml) of pyridazine in 1 ml of demineralized water.The reaction mixture was heated in a sealed glass vessel at 388 K for 2 d, leading to the formation of crystals suitable for single-crystal X-ray analysis.An IR spectrum of the title compound can be found in Fig. S5.

Experimental details
The IR spectrum was measured using an ATI Mattson Genesis Series FTIR Spectrometer, control software: WINFIRST, from ATI Mattson.
The PXRD measurement was performed with Cu K� 1 radiation (� = 1.540598A ˚) using a Stoe Transmission Powder Diffraction System (STADI P) equipped with a MYTHEN 1K detector and a Johansson-type Ge(111) monochromator.
Thermogravimetry and differential thermoanalysis (TG-DTA) measurements were performed in a dynamic air atmosphere in Al 2 O 3 crucibles using a STA-PT 1000 thermobalance from Linseis.The instrument was calibrated using standard reference materials.
Magnetic measurements were performed using a Quantum Design PPMS equipped with a 7 T magnet, using samples mounted in a gelatine capsule.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.The C-H hydrogen atoms were positioned with idealized geometry and refined as riding atoms with U iso (H) = 1.2 U eq (C).

Figure 3
Figure 3Arrangement of the chains in the crystal structure of the title compound with view along the crystallographic a-axis direction.